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Subject: Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: eranian@gmail.com
Cc: Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@elte.hu>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
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Date: Thu, 20 Aug 2009 12:24:32 +0200
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On Thu, 2009-08-20 at 12:13 +0200, stephane eranian wrote:
> On Wed, Aug 19, 2009 at 2:56 PM, Peter Zijlstra<a.p.zijlstra@chello.nl> wrote:
> > On Wed, 2009-08-19 at 22:36 +1000, Paul Mackerras wrote:
> >> Peter Zijlstra writes:
> >>
> >> > Provide the ability to configure a counter to send its output to
> >> > another (already existing) counter's output stream.
> >>
> >> What sort of thing might this be useful for?
> >
> > Some people complained that its tedious to mmap() for every counter and
> > would like to share the mmap() output buffer between counters.
> >
> 
> > This saves on address space and mlock budget and I guess fd management
> > logic.
> >
> Interesting to see, you seem to have changed your mind on this.

Lets say I haven't quite made up my mind yet, but people seem to want
it.

> I recall pointing this out in my early comments.

Yeah :-)

> But anyway, here are some more comments:
> 
>    - how does this work with the remapped counts?
>      Probably only see the count for the target, i.e., output,  event

Ah, right you are, we'd have to allow mmap() of the first page but no
data section.

>    - if samples from multiple events end up in the same buffer, how do
> I tell them apart,
>      i.e., how do I know sample X came from event A, sample X from event B?
>      This may be useful to detect patterns.

PERF_SAMPLE_ID?